A tandem scan system using separate probes for transmission and reception which are placed in different positions is employed for detecting flaws in an object being inspected which has a complicated shape, such as a turbine disc, and an apparatus which is provided with these separate probes for transmission and reception is called a two-probe ultrasonic flaw detection apparatus. In this conventional two-probe flaw detection apparatus, the reception probe is positioned with respect to the transmission probe so that it is on the acoustic axis of the ultrasonic beam (in the direction in which the intensity of the incident ultrasonic beam is a maximum) which are transmitted from the transmission probe into the object being inspected and are reflected therefrom, based on the concept of geometric optics, as shown in, for instance, column 2, line 15, to column 4, line 3, of Japanese Publication of Patent Application No. 58-26550 (1983) "Method and apparatus of ultrasonic flaw detection using two-probes".
However, in the prior art, the intensity of the received ultrasonic wave is not always a maximum at the position of the geometric-optical reflection of the acoustic axis, depending on the shape of the object 1 being inspected and the angle of incidence of the ultrasonic waves to the object. Any difference between the intensity at that position and the maximum thereof depends on the thickness of the object, and this difference increases with the thickness. Consequently, the conventional apparatus in which the reception probe is placed at the position of the geometric-optical reflection of the acoustic axis, in the ultrasonic detection of flaws in a thick-wall material, has the problem that the reception intensity decreases, and thus the sensitivity of detection of flaws drops.